Process for electrolytic preparation of quaternary ammonium compounds



United States Patent 3,523,068 PROCESS FOR ELECTROLYTIC PREPARATION OF QUATERNARY AMMONIUM COMPOUNDS Roy J. Eisenhauer, Pensacola, Fla., and Larry D. Gilmartin, Decatur, Ala., assignors to Monsanto Company, St. Louis, Mo., a corporation of Delaware No Drawing. Filed Dec. 19, 1966, Ser. No. 602,592 Int. Cl. B01k N00 US. Cl. 20472 7 Claims ABSTRACT OF THE DISCLOSURE Economical process for preparing quaternary ammonium hydroxide by electrolytic process from aqueous solution of tetraalkyl ammonium salts containing non-electrolyzable anions selected from the group consisting of sulfate, bisulfate, alkylsulfate, nitrate, carbonate, and bicarbonate.

This invention relates to a process for preparing quaternary ammonium hydroxides. More specifically, the invention relates to an electrolytic process for preparing tetraalkylammonium hydroxides from an aqueous solution of a tetraalkylammonium salt containing a non-electrolyzable anion.

Recently, it has been demonstrated that adiponitrile, an important chemical intermediate, can be prepared in commercially feasible yields by electrolytic hydrodirnerization of acrylonitrile in a single step synthesis. Generally, the electrolytic hydrodimerization reaction is effected in a cathode chamber of a two chamber electrolytic cell wherein a mixture of acrylonitrile, water, and an electrolyte are circulated in the cathode chamber and a product is withdrawn continuously.

Successful operation of this and other similar types of electrolytic organic synthesis processes is dependent upon the electrolyte. That is, the electrolyte must have the property of increasing the solubility of organic precursors and products in Water so that homogeneous solutions are present as catholytes and anolytes. Also, the electrolyte must have the property of providing sufficient ions in solution for electrical conductivity as well as an environment which will favor the desired synthesis. In the case of the electrolytic hydrodimerization of acrylonitrile, aqueous solutions of quaternary ammonium salts of sulfonic and carboxylic acids have been found to fulfill these requirements.

Also, it is generally desirable to control the pH of the catholyte or anolyte wherein the electrolytic synthesis is taking place. This control may be accomplished easily by the addition of an acid or base corresponding to the salt used to prepare the aqueous electrolyte. For example, toluenesulfonic acid and tetramethylammonium hydroxide may be used advantageously to control the pH of an aqueous catholyte solution prepared from tetramethylammonium toluenesulfonate for use in the electrolytic hydrodimerization of acrylonitrile.

Commercial electrolytic organic processes require that a practical and economically feasible method be provided for the synthesis of quaternary ammonium hydroxides which may be used to prepare quaternary ammonium salts for electrolytes and for necessary pH control.

It is an object of this invention to provide a novel process for the preparation of quaternary ammonium hydroxides in a simple and feasible commercial manner.

It is another object of this invention to provide a novel process for preparing tetraalkylammonium hydroxides in aqueous solutions without having toxic fumes emitting therefrom.

Still further, it is an object of this invention to provide a novel process for electrolytically preparing tetraalkylammonium hydroxides from tetraalkylammonium salts containing non-electrolyzable anions.

Other objects of this invention will become apparent as the invention is fully developed within the specification.

These and other objects of this invention are accomplished by providing an electrolytic process for preparing an aqueous solution of tetraalkylammonium hydroxide in an electrolytic cell, the electrolytic cell having at least one anolyte chamber with anode separated by a cation permselective membrane from at least one catholyte chamber with cathode, from an anolyte feed comprised of an aqueous solution containing from about 10% to about 50% by weight of a tetraalkylammonium salt containing a nonelectrolyzable anion and a catholyte feed comprised of water, comprising in combination:

(a) Passing direct electric current by means of the anode and the cathode through said anolyte feed and said catholyte feed while the anolyte feed is positioned in the at least one anolyte chamber between said anode and said membrane and the catholyte feed is positioned in the at least one catholyte chamber between said cathode and said membrane;

(b) Controlling the flow of said anolyte feed to the at least one anolyte chamber at a rate from about 1.5 lbs. to about 4.0 lbs. per hour per ampere of said direct electric current;

(c) Controlling the flow of said catholyte feed to the at least one catholyte chamber at a rate of from about 1.4 lbs. to about 4.0 lbs. per hour per ampere of said direct electric current;

(d) Collecting a portion of the catholyte exiting from the at least one catholyte chamber as the aqueous solution of tetraalkylammonium hydroxide.

The application of direct electric current through the electrolytic cell causes the tetraalkylammonium ions to migrate through the cation permselective membrane to the catholyte chamber resulting in the formation of tetraalkylammonium hydroxide and hydrogen gas at the cathode and oxygen gas at the anode. The anions of the tetraalkylammonium salt in the anolyte feed are non-electrolyzable and thus do not participate in the electrolytic process but accumulate as the acid form in the anolyte stream.

The apparatus necessary for the process'of this invention comprises one or more electrolytic cells of a type well known in the art which have an anode and a cathode and at least two chambers between the anode and the cathode separated by a cation permselective membrane. The anode may be constructed of graphite, carbon, platinum, lead and other like materials known in the art; however it is preferred that the anode be constructed of a material principally composed of lead. The cathode may be constructed of any electrical conducting material and preferably one having a low hydrogen overvoltage so that the cell can be operated at minimum electrical power requirements.

As mentioned previously, the membrane is of the cation perm-selective type. The membrane should be constructed so that it will prevent catholyte contamination of the tetraalkylammonium salts While not imparting undue resistance to cation flow. Also, the membrane should be constructed of material which will have reasonable hydraulic resistance, e.g. the membrane can be constructed of Teflon filter cloth to impart strength thereto.

The electrolytic process for the production of tetraalkylammonium hydroxides may be operated on a once through How of anolyte and catholyte or the process may be operated on a continuous basis through a series of electrolytic cells constructed and arranged so that the flow of anolyte and catholyte may be either in series or in parallel through the group of electrolytic cells. One or both of the electrolytic streams may be recirculated through one or more cells with continuous or batchwise withdrawal of product with a provision for adding tetraalkylarnmonium salts to maintain the desired concentration in the anolyte feed stream and the addition of water to the catholyte feed to maintain the desired volume of catholyte. The use of recirculation of anolyte and catholyte may be desired to permit the removal of heat generated in the electrolytic cell.

In the electrolytic process in accordance with this invention, it has been found that the desired concentration of the tetraalkylammonium salts in the anolyte be in a concentration range of from about to about 50% by weight. Concentrations of the tetraalkylammoniurn salts below 10% exhibit poor electrical conductivity whereas concentrations in excess of 50% are less water soluble and are likely to precipitate out of solution.

The tetraalkylammonium salts are defined as salts containing a nonelectrolyzable anion. Such useful salts include tetraalkylammonium salts containing an anion selected from the group consisting of nitrate, carbonate, bicarbonate, sulfate, alkylsulfate wherein the alkyl contains from about 1 to about 3 aliphatic carbon atoms, and bisulfate and the alkyl grouping within the tetraalkylammonium ion contains from about 1 to about 3 aliphatic carbon atoms. Preferable tetraalkylammonium salts include tetraalkylarnmonium sulfate, tetraalkylammonium akylsulfate and a combination of these two. More preferably, the tetraalkylammonium salts can be tetraalkylammonium sulfate, tetraalkylammonium ethylsulfate, and a combination of the two. Other useful tetraalkylammonium salts include salts containing anions which are non-electrolyzable and which are water soluble, such salts being of like character to those enumerated above.

The inlet temperature of the anolyte and catholyte entering the electrolytic cell, the current flowing through the cell, and the voltage drop across the anolyte and catholyte chambers of the cell are interdependent variables which can be controlled. The flow rate of the anolyte and the catholyte through the anolyte and catholyte chambers should be adjusted so that any combination of the above stated interdependent variables does not cause the anolyte temperature within the cell to exceed about 50 C. (at higher temperatures the corrosive nature of the anolyte is greatly increased) and the catholyte temperature to exceed about 50 C. (at higher temperatures the catholyte emits fumes smelling of ammonia). Also, the flow rate of the anolyte through the anolyte chamber should be sufiicient to sweep the oxygen formed during the electrolytic processes away from the anode. However, the rate should not be sufficiently high to cause erosion or like damage to be efiected upon the anode. The catholyte feed rate should be sufiicient to sweep the hydrogen formed during the electrolyte process away from the cathode area. It has been found that anolyte feed rates in the anolyte chamber within the range of from about 1.5 lbs. to about 4.0 lbs. per hour per ampere of the direct electric current are useful flow rates with the invention; preferably, the anolyte feed rate can be within the range of from about 2.0 lbs. to about 3.0 lbs. per hour per ampere of the direct electric current. The flow rate of the catholyte feed through the catholyte chamber can be within the range of from about 1.4 lbs. to about 4.0 lbs. per hour per ampere of the direct electric current and, prefer ably, it can be within the range of from about 1.6 lbs. to about 2.5 lbs. per hour per ampere of the direct electric current. The above indicated flow rates are suificient to give economical results with the electrolytic process of this invention and to keep the oxygen and hydrogen swept away from the anode and cathode, respectively. When the electrolytic cell is operated at the high flow rates, recirculation of the anolyte and catholyte through the cell may be used so that suitable external means, e.g. heat exchangers, may be provided for cooling the anolyte and catholyte individually prior to their reentry into the electrolytic cell.

As mentioned previously, the anions of the tetraalkylammonium salts are non-electrolyzable. As a result, these anions tend to accumulate in the anolyte feed stream. Where the anions exhibit a corrosive attack on the anode and materials which they are in contact, it is preferred that the anolyte feed be adjusted, i.e. the anion concentration in the anolyte be reduced to a pH of not less than about 0.3.

In operation of the process in accordance with this invention using recirculation of the catholyte, a portion of the catholyte is withdrawn as the product, which is an aqueous solution of tetraalkylamrnonium hydroxide. The rate at which the product is withdrawn affects the concentration of the desired tetraalkylammonium hydroxide in the aqueous product solution. Pure water is added to the catholyte feed stream, thus diluting the stream, to replace this product withdrawn so that a substantially constant catholyte volume can be maintained. In general, the greater the portion of the product stream withdrawn from the exiting catholyte stream, the lower the concentration of the desired tetraalkylamrnonium hydroxide in the prod uct stream. Since the electrical resistance across the catholyte increases with lowering concentration of tetraalkylammonium hydroxide in the catholyte and current efliciency decreases with increasing concentration of tetraalkylammonium hydroxide in the catholyte, it has been found that the rate of product withdrawal from the exiting catholyte stream should be controlled to maintain a tetraalkylammonium hydroxide concentration of from about 6% to about 14%, by weight, in the aqueous catholyte feed stream to the eletcrolytic cell.

In accordance with the process of this invention as outlined above and where the electrolytic process uses recirculating catholyte and anolyte, the current efficiency of the process is from about 35% to about 60%. Where the aqueous solution of the tetraalkylarnmonium hydroxide is contaminated with small amounts of the precursor, i.e. traces of the tetraalkylammonium salts have leaked through the membrane, it can be passed through a bed of anion exchange resin wherein the anion of the tetraalkylammonium salt is exchanged for an hydroxy anion.

The product of this invention, i.e. the aqueous solution of 'tetraalkylammonium hydroxide, can be a tetraalkylammonium hydroxide wherein the alkyl grouping contains from about 1 to about 3 aliphatic carbon atoms. The desired tetraalkylammonium hydroxide will depend upon the precursor used in the anolyte feed stream, that is if a tetraethylammonium hydroxide is desired as the product then a tetraethylammonium salt is necessary in the anolyte feed stream to the electrolytic cell.

The following example is presented to specifically illustrate working embodiments of this invention.

EXAMPLE I Tetraethylammonium hydroxide is prepared in an electrolytic cell unit having 4 cells, each cell having a graphite anode, a stainless steel cathode and a cation permselective membrane separating the anolyte chamber in the anode from the catholyte chamber in the cathode. The cells are electrically connected in series and the anode, the cathode and membrane are in a vertical position. The anode within the cell is 3.4 ins. wide, 3.6 ins. high, and 0.75 in. deep. The cathode within the cell is 3.4 ins. wide, 3.6 ins. high, and 0.25 in. deep. Measurements of the membrane are 4.4 ins. wide, and 4.6 ins. high. Each cell unit has an anolyte inlet, an anolyte outlet, a catholyte inlet and a catholyte outlet, the flow of which constitutes a parallel feed arrangement to the four cells with the anolyte and catholyte entering the bottom of their respective chambers and flowing up the face of the membrane and out the top of the cell units. Anolyte at a pH of about 1.4 and containing 25% by weight of tetraethylammonium ion, 17.8% by weight of tetraethylammonium sulfate, and 23.5% by weight of tetraethylammonium ethylsulfate, is circulated at a rate of 500 lbs. per hour through the cell unit. In

starting the unit, the catholyte feed stream is composed of about 100% water but as the electrolytic process developes the catholyte feed stream contains about of tetraethylammonium hydroxide and about 90% of water. The catholyte feed stream is circulated at a rate of 460 lbs. per hour through the catholyte chambers. As the electrolytic process operates, additional tetraethylammonium ion solution is added to the anolyte stream to maintain a concentration of about by weight of the tetraethylammonium ion and 40 lbs. per hour of water is added to the catholyte feed stream to maintain the circulation of a volume rate of about 460 lbs. per hour. The temperature of the anolyte feed stream is about 45 C. and the temperature of the catholyte feed stream is about 45 C. A current of 180 to 250 amperes is passed through the cell unit and the potential drop across the cell unit is measured at 20 to volts. Oxygen and hydrogen are formed at the anode and cathode respectively, and these gases are withdrawn from the cell unit. A product of 40 lbs. per hour is collected from the exiting catholyte stream, and upon analysis, is found to contain 10% by weight of tetraethylammonium hydroxide. Under the conditions of this cell unit, 55% of the tetraethylammonium ion in the anolyte stream is converted to tetraethylammonium hydroxide with a current etficiency of about to about As is readily ascertainable from the above example and the preceeding description, the electrolytic preparation of an aqueous solution of tetraalkylammonium hydroxide can be accomplished in accordance with this invention in a simple and economical manner. It is to be understood that the invention is not to be restricted in view of the above example but the invention can be practiced with as many widely different embodiments and operating conditions without parting from the spirit and scope of this invention.

What is claimed is:

1. An electrolytic process for preparing an aqueous solution of tetraalkylammonium hydroxide in an electrolytic cell, the electrolytic cell having at least one anolyte chamber with anode separated by a cation permselective membrane from at least one catholyte chamber with cathode, from an anolyte feed comprised of an aqueous solution of from about 10% to about by weight of a tetraalkylammonium salt containing a non-electrolyzable anion and a catholyte feed comprised of water, comprising in combination:

(a) passing direct electric current by means of the cathode through said anolyte feed and said catholyte feed while the anolyte feed is positioned in the at least one anolyte chamber between said anode and said membrane and the catholytic feed is positioned in the at least one catholyte chamber between said cathode and said membrane;

(b) controlling the flow of said anolyte through the at least one anolyte chamber at a rate of from about 1.5 lbs. to about 4 lbs. per hour per ampere of said direct electric current;

(c) controlling the flow of said catholytic feed through the at least one catholyte chamber at a rate of from about 1.4 lbs. to about 4.0 lbs. per hour per am pere of said direct electric current;

((1) recirculating the catholyte feed and the anolyte feed while maintaining from about 6% to about 14% by weight tetraalkylammonium hydroxide in said catholyte feed at the at least one catholyte chamber; and

(e) collecting a portion of the catholyte exiting the at least one catholyte chamber as an aqueous solution of tetraalkylammonium hydroxide.

2. The process of claim 1 wherein the anolyte feed is comprised of an aqueous solution containing from about 15% to about 35% of the tetraalkylammonium salt.

3. The process of claim 1 wherein the anion of the tetraalkylammonium salt is selected from the group consisting of sulfate, bisulfate, alkylsulfates, nitrate, carbonate, and bicarbonate.

4. The process of claim 1 wherein the tetraalkylam- 'monium salt is selected from the group consisting of tetra ethylammonium sulfate, tetraethylammonium ethylsulfate, and a combination of tetraethylammonium sulfate and tetraethylammonium ethylsulfate.

5. The process of claim 1 wherein the anolyte feed flow rate is from about 2 lbs. to about 3 lbs. per hour per ampere of said direct electric current.

6. The process of claim 1 wherein the catholyte feed rate is from about 1.6 lbs. to about 2.5 lbs. per hour per ampere of said direct electric current.

7. An electrolytic process for preparing an aqueous solution of tetraalkylammonium hydroxide in an electrolytic cell, the electrolytic cell having at least one anolyte chamber with anode separated by a cation permselective membrane from at least one catholyte chamber with cathode, from an anolyte feed comprised of an aqueous solution containing from about 15% to about 35% by weight of a tetraethylammonium salt selected from the group consisting of tetraethylammonium sulfate tetraethylammonium ethylsulate and a combination of tetraethylammonium sulfate and tetraethylammonium ethylsulfate, and a catholyte feed comprised of water, comprising in combination:

(a) passing direct electric current by means of the anode and the cathode through said anolyte feed and catholyte feed while the anolyte feed is positioned in the at least one anolyte chamber between said anode and said membrane and the catholyte feed is positioned in the at least one catholyte chamber between said cathode and said membrane;

(b) controlling the flow of said anolyte feed through the at least one anolyte chamber at a rate of from about 2 lbs. to about 3 lbs. per hour per ampere of said direct electric current;

(c) controlling the flow of said catholyte feed through the at least one catholyte chamber at a rate of from about 1.6 lbs. to about 2.5 lbs. per hour per ampere of said direct electric current;

((1) recirculating the catholyte feed and the anolyte feed while maintaining from about 6% to about 14% by weight tetraalkylammonium hydroxide in said catholyte feed at the at least one catholyte chamber; and

(e) collecting a portion of the catholyte exiting from the at least one catholyte chamber as an aqueous solution of tetraalkylammonium hydroxide.

References Cited UNITED STATES PATENTS 2,363,387 11/1944 Bock 20472 2,737,486 3/1956 Bodamer 204-72 2,967,806 1/1961 Osborne et a1 20472 3,193,480 7/1965 Baizer et al 204-72 XR PATRlCK P- G RVIN, Primary Examiner 

